1. Field of the Invention
The present invention relates to an ink composition suitable for ink jet recording.
2. Background Art
It is important that an image formed by an ink composition possesses good waterproofness and lightfastness and has no significant feathering or bleeding.
In order to realize good waterproofness, a combination of a cationic resin with an anionic dye has hitherto been used in the art.
For example, Japanese Patent Laid-Open No. 119280/1987 discloses an ink comprising a hydroxyethylated polyethylene-imine polymer and a dye component. This publication describes that the combination of the polyethylene-imine polymer with the dye component can develop waterproofness.
Japanese Patent Publication No. 91494/1995 discloses an ink composition comprising a hydroxyalkylated polyallylamine polymer and a dye. This publication describes that the combination of the primary amine with the dye can develop waterproofness.
For the above two inks, a further improvement in ensuring satisfactory waterproofness of the image is required because the polymer has been substituted by a highly hydrophilic hydroxyalkyl group.
Japanese Patent Laid-Open Nos. 255876/1990, 296876/1990, and 188174/1991 disclose an ink composition comprising a polyamine, having a primary amine group, with a molecular weight of not less than 300, an anionic dye, and a stabilizer. These publications describe that the combination of the primary amine with the anionic dye can develop waterproofness.
Japanese Patent Laid-Open No. 305011/1995 discloses a water-base ink comprising a basic, water-soluble polymer, an anionic dye with a volatile base as a counter ion, and a buffer with a volatile base as a counter ion. This publication describes that the volatile base inhibits the dissociation of the polymer in the ink and, on paper, the volatile base is evaporated to promote a salt formation reaction between the polymer and the dye, thereby developing waterproofness.
Japanese Patent Laid-Open No. 238783/1987 discloses an ink jet recording sheet containing a homopolymer of a diallylaminate and a monoallylaminate or a copolymer of the diallylaminate with the monoallylaminate. This publication describes that an insolubilizing reaction between the polymer and the dye is created on this recording medium to develop waterproofness.
For some of the above ink compositions, a further improvement in lightfastness is required. For example, in the case of ink compositions using polyethyleneimine, which is specifically described as the cationic resin in Japanese Patent Laid-Open Nos. 255876/1990, 296876/1990, and 188174/1991, this resin strongly attacks the dye, and, for example, when these inks are allowed to stand at a high temperature, this results in decomposition of the dye or accelerates the photodecomposition of the print to often render the lightfastness of the ink lower than that of the ink using the dye alone.
Japanese Patent Laid-Open No. 305011/1995 discloses a recording method using the recording medium containing a cationic resin. Since the ink composition disclosed in the publication is not waterproof, the waterproofness cannot be developed in an image when recording media other than the recording medium described in this publication are used.
For the printed image, what is further important is to prevent feathering or bleeding, especially color-to-color bleeding in the case there inks of two or more colors are used.
The present inventors have now found that an ink composition containing a cationic, water-soluble resin having a specific structure can realize an image possessing good waterproofness and lightfastness and having no significant feathering or bleeding. The present invention has been made based on such finding.
It is therefore an object of the present invention to provide an ink composition which can yield an image possessing good waterproofness and lightfastness and having no significant feathering or bleeding, especially no significant color-to-color bleeding.
Furthermore, it is an object of the present invention to provide an ink composition suitable for ink jet recording.
According to one aspect of the present invention, there is provided an ink composition comprising at least an alkali-soluble colorant, a water-soluble organic solvent, water, and a cationic, water-soluble resin,
the cationic, water-soluble resin comprising a repeating unit represented by the following formula (I): 
wherein R1 and R2 which may be the same or different represent a hydrogen atom or a C1-5 alkyl group, provided that R1 and R2 do not simultaneously represent a hydrogen atom: and
n is 0, 1, or 2.
Ink Composition
The ink composition according to the present invention may be used in recording methods using an ink composition. Recording methods using an ink composition include, for example, an ink jet recording method, a recording method using writing utensils, such as pens, and other various recording methods. In particular, the ink composition according to the present invention is preferably used in the ink jet recording method.
The ink composition according to the present invention contains basically alkali-soluble colorant, a water-soluble organic solvent, water, and a cationic, water-soluble resin, the cationic, water-soluble resin comprising a repeating unit represented by the formula (I).
According to the present invention, the cationic, water-soluble resin is stably dissolved in the ink composition. Once the ink composition deposits onto a recording medium, the cationic, water-soluble resin electrostatically interacts with a colorant, and, in addition, the resin also interacts with the recording medium. Therefore, the resin is stably fixed on the recording medium. It is believed that the fixation of the resin on the recording medium permits the colorant, together with the resin, to be fixed on the recording medium, thus imparting waterproofness to the printed image. It is also believed that an image having no significant feathering or bleeding is realized as a result of firm fixation of the colorant on the recording medium.
For the cationic, water-soluble resin used in the present invention, R1 and R2 in the formula (I) may be the same or different and represent a hydrogen atom or a C1-5 alkyl group, preferably a methyl group. However, R1 and R2 do not simultaneously represent a hydrogen atom. Most preferably, R1 and R2 simultaneously represent a methyl group.
In the formula (I), n is 0, 1, or 2, preferably 1.
According to a preferred embodiment of the present invention, the average molecular weight of the cationic, water-soluble resin is preferably about 300 to 10,000, more preferably about 400 to 5,000, most preferably about 500 to 2,500.
According to a preferred embodiment of the present invention, the cationic, water-soluble resin may comprise units other than the repeating unit represented by the formula (I). This can improve properties of the cationic, water-soluble resin. Monomers for providing units which may be contained in the resin include ethylene, propylene, isobutylene, styrene, vinyl chloride, vinylidene chloride, vinyl alcohol, vinylamine, vinyl acetate, acrylic acid or acrylic esters (for example, lower alkyl esters), methacrylic acid or methacrylic esters (for example, lower alkyl esters), acrylonitrile, methyl vinyl ether, vinylpyrrolidone, allylamine, diallylamine, and sulfur dioxide. Although the amount of units, derived from these monomers, present in the cationic, water-soluble resin is not particularly limited, it is preferably not more than 70%, more preferably not more than 30%, in terms of the molar ratio. In the cationic, water-soluble resin, these units may be present in a block form or a random form.
According to a preferred embodiment of the present invention, the ink composition of the present invention may further comprise a water-soluble resin other than the cationic, water-soluble resin. The water-soluble resin other than the cationic, water-soluble resin is preferably a nonionic, water-soluble resin, and examples of nonionic, water-soluble resins usable herein include polyacrylamide, polymethacrylic acid hydroxy esters, such as polyhydroxyethyl methacrylate, polyvinyl pyrrolidone, polyvinyl alcohol, and polyethylene glycol. Addition of these water-soluble resins can further stabilize the ink composition.
The alkali-soluble colorant contained in the ink composition according to the present invention may be either a dye or a pigment. The term xe2x80x9calkali-solublexe2x80x9d used herein means xe2x80x9csoluble in an alkaline medium.xe2x80x9d The water-soluble group contained in the molecule may be an acidic or basic dissociable group or non-dissociable functional group, or alternatively may be a group containing a plurality of the above groups. Further, the colorant may be one soluble in an acidic solution so far as it is soluble in an alkali.
While the content of the colorant may not be limited, it is preferably 0.5 to 20% by weight based on the total weight of the ink composition. The colorant in the above content range can realize a printed image having satisfactory optical density. Further, the viscosity of the ink composition containing the colorant in the above content range can be easily modified to a value suitable for ink jet recording.
More preferably, the colorant is selected from organic dyes or organic pigments which are suitable because of high color density per unit weight and bright color.
Useful dyes are organic colored materials soluble in water and fall into categories of acidic dyes, direct dyes, reactive dyes, soluble vat dyes, and food dyes according to the color index. Further, colorants, insoluble in neutral water, falling within categories of oil-soluble dyes and basic dyes according to the color index may also be used so far as they are soluble in an aqueous alkali solution.
Preferably, the pigment is selected from those falling into the category of pigments according to the color index. Pigments are generally recognized as water-insoluble, organic colored materials, some of which are soluble in an alkali and usable in the present invention.
Examples of dyes and pigments usable herein include: yellow dyes and pigments, such as C.I. Acid Yellow 1, 3, 11, 17, 19, 23, 25, 29, 36, 38, 40, 42, 44, 49, 59, 61, 70, 72, 75, 76, 78, 79, 98, 99, 110, 111, 127, 131, 135, 142, 162, 164, and 165, C.I. Direct Yellow 1, 8, 11, 12, 24, 26, 27, 33, 39, 44, 50, 55, 58, 85, 86, 87, 88, 89, 98, 110, 132, 142, and 144, C.I. Reactive Yellow 1, 2, 3, 4, 6, 7, 11, 12, 13, 14, 15, 16, 17, 18, 22, 23, 24, 25, 26, 27, 37, and 42, C.I. Food Yellow 3 and 4, C.I. Solvent Yellow 15, 19, 21, 30, and 109, and C.I. Pigment Yellow 23; red dyes and pigments, such as C.I. Acid Red 1, 6, 8, 9, 13, 14, 18, 26, 27, 32, 35, 37, 42, 51, 52, 57, 75, 77, 80, 82, 85, 87, 88, 89, 92, 94, 97, 106, 111, 114, 115, 117, 118, 119, 129, 130, 131, 133, 134, 138, 143, 145, 154, 155, 158, 168, 180, 183, 184, 186, 194, 198, 209, 211, 215, 219, 249, 252, 254, 262, 265, 274, 282, 289, 303, 317, 320, 321, and 322, C.I. Direct Red 1, 2, 4, 9, 11, 13, 17, 20, 23, 24, 28, 31, 33, 37, 39, 44, 46, 62, 63, 75, 79, 80, 81, 83, 84, 89, 95, 99, 113, 197, 201, 218, 220, 224, 225, 226, 227, 228, 229, 230, and 231, C.I. Reactive Red 1, 2, 3, 4, 5, 6, 7, 8, 11, 12, 13, 15, 16, 17, 19, 20, 21, 22, 23, 24, 28, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 45, 46, 49, 50, 58, 59, 63, and 64, C.I. Solubilized Red 1, C.I. Food Red 7, 9, and 14, and C.I. Pigment Red 41, 48, 54, 57, 58, 63, 68, and 81; blue dyes and pigments, such as C.I. Acid Blue 1, 7, 9, 15, 22, 23, 25, 27, 29, 40, 41, 43, 45, 54, 59, 60, 62, 72, 74, 78, 80, 82, 83, 90, 92, 93, 100, 102, 103, 104, 112, 113, 117, 120, 126, 127, 129, 130, 131, 138, 140, 142, 143, 151, 154, 158, 161, 166, 167, 168, 170, 171, 182, 183, 184, 187, 192, 199, 203, 204, 205, 229, 234, 236, and 249, C.I. Direct Blue 1, 2, 6, 15, 22, 25, 41, 71, 76, 77, 78, 80, 86, 87, 90, 98, 106, 108, 120, 123, 158, 160, 163, 165, 168, 192, 193, 194, 195, 196, 199, 200, 201, 202, 203, 207, 225, 226, 236, 237, 246, 248, and 249, C.I. Reactive Blue 1, 2, 3, 4, 5, 7, 8, 9, 13, 14, 15, 17, 18, 19, 20, 21, 25, 26, 27, 28, 29, 31, 32, 33, 34, 37, 38, 39, 40, 41, 43, 44, and 46, C.I. Solubilized Vat Blue 1, 5, and 41, C.I. Vat Blue 29, C.I Food Blue 1 and 2, C.I. Basic Blue 9, 25, 28, 29, and 44, and C.I. Pigment Blue 1 and 17; and black dyes and pigments, such as C.I. Acid Black 1, 2, 7, 24, 26, 29, 31, 48, 50, 51, 52, 58, 60, 62, 63, 64, 67, 72, 76, 77, 94, 107, 108, 109, 110, 112, 115, 118, 119, 121, 122, 131, 132, 139, 140, 155, 156, 157, 158, 159, and 191, C.I. Direct Black 17, 19, 22, 32, 35, 38, 51, 56, 62, 71, 74, 75, 77, 94, 105, 106, 107, 108, 112, 113, 117, 118, 132, 133, 146, 154, 168, 171, and 195, C.I. Reactive Black 1, 3, 4, 5, 6, 8, 9, 10, 12, 13, 14, and 18, C.I. Solubilized Vat Black 1, and C.I. Food Black 2. These colorants may be used alone or as a mixture of two or more.
In the ink composition according to the present invention, water is a main solvent. Water may be pure water obtained by ion exchange, ultrafiltration, reverse osmosis, distillation or the like, or ultrapure water. Further, water, which has been sterilized by ultraviolet irradiation or by addition of hydrogen peroxide, is suitable because, when the ink composition is stored for a long period of time, it can prevent the growth of mold or bacteria.
The ink composition according to the present invention may further comprise a base. Examples of bases usable herein include: inorganic bases, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, barium hydroxide, strontium hydroxide, radium hydroxide, beryllium hydroxide, magnesium hydroxide, and ammonia; mono-, di-, and tri-lower alkylamines, such as ethylamine, diethylamine, triethylamine, propylamine, dipropylamine, diisopropylamine, tert-butylamine, dibutylamine, diisobutylamine, isopropylamine, sec-butylamine, and pentylamine; lower alkyl lower hydroxyalkoxyamines, such as 3-ethoxypropylamine and 3-methoxypropylamine; lower alkyl lower alkoxyamines, such as 3-ethoxypropylamine and 3-methoxypropylamine; mono-, di-, and tri-lower hydroxyalkylamines, such as 2-aminoethanol, 2-(dimethylamino)ethanol, 2-(diethylamino)ethanol, diethanolamine, N-butyldiethanolamine, triethanolamine, aminomethylpropanol, and triisopropanolamine; and organic amines, such as iminobispropylamine, 3-diethylaminopropylamine, dibutylaminopropylamine, methylaminopropylamine, dimethylaminopropanediamine, and methyliminobispropylamine. These bases can stably dissolve the cationic, water-soluble resin and the colorant and can maintain the dissolved state. For example, when an acid addition salt is used as the cationic, water-soluble resin, the alkali-soluble colorant is in some cases insoluble. Addition of the base enables the alkali-soluble colorant to be stably dissolved.
In the ink composition according to the present invention, the water-soluble organic solvent means to a medium having an ability to dissolve a solute and is preferably selected from water-soluble solvents having a smaller vapor pressure than water. Examples thereof include: polyhydric alcohols, such as ethylene glycol, propylene glycol, butanediol, pentanediol, 2-butene-1,4-diol, 2-methyl-2,4-pentanediol, glycerin, 1,2,6-hexanetriol, diethylene glycol, and dipropylene glycol; ketones, such as acetonylacetone; esters, such as xcex3-butyrolactone, diacetin, and triethyl phosphate; lower alkoxy lower alcohols, such as 2-methoxyethanol and 2-ethoxyethanol; furfuryl alcohol; tetrahydrofurfuryl alcohol; and thiodiglycol. Further, solvents usable herein include not only those that are liquid at room temperature but also those that are solid at room temperature and, when heat melted, can function as a solvent, and those that, when used in combination with an aqueous solution or other solvent(s), can function as a solvent. That the vapor pressure of the organic solvent is lower than that of pure water is advantageous in that, even though drying of the ink proceeds at the front end of the ink jet head, the organic solvent can stably dissolve the solutes.
Although the amount of the water-soluble organic solvent used may be suitably determined, it is preferably, for example, 5 to 50% by weight based on the total weight of the ink.
According to a preferred embodiment of the present invention, the ink composition according to the present invention may further comprise an additional organic solvent. Examples of additional solvents usable herein include imidazole, methylimidazole, hydroxyimidazole, triazole, nicotinamide, dimethylaminopyridine, xcex5-caprolactam, 1,3-dimethyl-2-imidazolidinone, lactamide, sulfolane, dimethylsulfoxide, 1,3-propanesultone, methyl carbamate, ethyl carbamate, 1-methylol-5,5-dimethylhydantoin, hydroxyethylpiperazine, piperazine, ethyleneurea, propyleneurea, ethylene carbonate, propylene carbonate, dimethyl sulfoxide, N-methyl-2-pyrrolidinone, 2-pyrrolidinone, acetamide, formamide, dimethylformamide, N-methylformamide, and dimethylacetamide. The addition of the organic solvent can prevent the creation of a precipitate, for example, at a low temperature and permits printing to be stably performed under such an environment.
If necessary, assistants commonly used in the ink for ink jet recording may be added to the ink composition according to the present invention. Examples of assistants usable herein include penetration accelerators, viscosity modifiers, surface tension modifiers, hydrotropy agents, humectants, pH adjustors, antimolds, chelating agents, preservatives, and rust preventives. When the ink is used in an ink jet recording method wherein the ink is charged, it is possible to add a specific resistance modifier selected from inorganic salts, such as lithium chloride, sodium chloride, and ammonium chloride.
Penetration accelerators usable herein include: lower alcohols, such as ethanol, isopropanol, butanol, and pentanol; cellosolves, such as ethylene glycol monobutyl ether; carbitols, such as diethylene glycol monobutyl ether, triethylene glycol monobutyl ether glycol ether; and surfactants.
Surface tension modifiers usable herein include diethanolamine, triethanolamine, alcohols, such as glycerin and diethylene glycol, and nonionic, cationic, anionic, or amphoteric surfactants.
Preferred hydrotropy agents usable herein include urea, alkylureas, ethyleneurea, propyleneurea, thiourea, guanidine acid salts, and tetraalkylammonium halides.
Humectants usable herein include glycerin and diethylene glycol which may be added also as the water-soluble organic solvent. Further humectants include saccharides, such as maltitol, sorbitol, gluconic lactone, and maltose.
Bases described above may be used also as the pH adjustor.
Ink set
According to another aspect of the present invention, there is provided an ink set comprising: a black ink comprising at least one alkali-soluble colorant selected from a group of dyes consisting of C.I. Direct Black 19, 35, 154, 168, 171, and 195 and C.I. Food Black 2; a yellow ink comprising at least one colorant selected from a group of dyes consisting of C.I. Direct Yellow 50, 55, 86, 132, 142, and 144 and C.I. Acid Yellow 23; a cyan ink comprising at least one colorant selected from a group of dyes consisting of C.I. Direct Blue 86, 87, and 199 and C.I. Acid Blue 9 and 249, and a magenta ink comprising at least one colorant selected from a group of dyes consisting of C.I. Direct Red 9 and 227, C.I. Acid Red 52 and 289, and dyes represented by the following structural formula (II). Use of this ink set can provide a full color image having excellent waterproofness, independently of the recording medium. Without intending to be bound by theory, it is believed that the above groups of dyes exhibit strong electrostatic interaction with the cationic, water-soluble resin comprising a repeating unit represented by the formula (I), imparting excellent waterproofness. 
wherein R3 and R4 represent a hydrogen atom, a C1-5 alkyl group, an aryl group, a C1-5 alkoxy group, or a phenoxy group or a derivative thereof, a triazine ring or a derivative thereof, a carboxyl group or a salt thereof, or a sulfonyl group or a derivative thereof;
X represents a hydrogen or halogen atom; and
Y1 and Y2 which may be the same or different represent an alkali metal atom, ammonium, or a C1-5 alkylammonium.
Among the group of dyes represented by the structural formula (II), dyes represented by the following structural formula (III) are preferred. These dyes fall into the category of C.I. Acid Red 249 according to the color index. 
Further, dyes represented by the following structural formula (IV) may also be preferably used: 
Use of the ink composition according to the present invention in combination with an ink composition containing an anionic material can realize an image having no significant color-to-color bleeding between two inks. It is believed that upon contact of the two ink compositions on a recording medium, the cationic, water-soluble resin contained in the ink composition according to the present invention is reacted with the anionic material to form a precipitate, inhibiting further spreading of the ink composition on the recording medium. This results in the realization of an image having no significant color-to-color bleeding.
Thus, according to another aspect of the present invention, there is provided an ink set comprising the ink composition of the present invention as a first ink composition and the ink composition containing an anionic material as a second ink composition.
The ink set generally comprises an yellow ink, a magenta ink, a cyan ink, and a black ink. According to a preferred embodiment of the present invention, the ink composition of the present invention constitutes a yellow ink, a magenta ink, and a cyan ink with the ink composition containing an anionic material constituting a black ink, or alternatively, the ink composition of the present invention constitutes a black ink with the ink composition containing an anionic material constituting a yellow ink, a magenta ink, and a cyan ink.
The second ink composition, that is, the ink composition containing an anionic material, basically comprises a colorant, a water-soluble organic solvent, water, an anionic material, and a base.
According to a preferred embodiment of the present invention, the anionic material may be an anionic resin. Preferred examples of anionic resins usable herein include those which has as a functional group an anionic group, such as a sulfonic acid, carboxylic acid, phosphoric acid, or hydroxyl group, and, when combined with a base to form a salt, can be made water-soluble. Specific examples thereof include: cellulose derivatives, such as carboxymethyl cellulose and viscose; naturally occurring polymers, such as alginic acid, gum arabic, tragacanth, and lignin sulfonic acid; starch derivatives, such as starch phosphate and carboxymethyl starch salts; and synthetic polymers, such as polyacrylic acid, polymethacrylic acid, polyvinylsulfuric acid, polyvinylsulfonic acid, condensed naphthalenesulfonic acid, ethylene/acrylic acid copolymer, styrene/acrylic acid copolymer, styrene/methacrylic acid copolymer, acrylic ester/acrylic acid copolymer, acrylic ester/methacrylic acid copolymer, methacrylic ester/acrylic acid copolymer, methacrylic ester/methacrylic acid copolymer, styrene/itaconic acid copolymer, itaconic ester/itaconic acid copolymer, vinylnaphthalene/acrylic acid copolymer, vinylnaphthalene/methacrylic acid copolymer, vinylnaphthalene/itaconic acid copolymer, phenolic resin and copolymers thereof.
In the present invention, the colorant in the second ink composition may be a pigment dispersed in the above anionic resin.
According to another preferred embodiment of the present invention, the anionic material may be a pigment having an anionic functional group on its surface. The anionic group combines with a base to form a salt, enabling pigment particles to be dispersed in water. Anionic functional groups usable herein include, for example, sulfonic acid, carboxylic acid, and phosphoric acid groups. The pigment having an anionic functional group on its surface may be prepared by grafting the functional group onto the surface of the pigment. Commercially available pigments may be used as the pigment, and examples thereof include Microjet CW-1 and Microjet CW-2 (tradename, manufactured by Orient Chemical Industries Ltd.).
Bases usable herein include those described above in connection with the ink composition according to the present invention.
The other components of the ink composition containing an anionic material of the ink set according to the present invention may be basically the same as those of the ink composition according to the present invention.